Protective tube for electrical elements and its method of manufacture

ABSTRACT

DISCLOSED HEREIN IS A PROTECTIVE TUBE FOR A THERMOCOUPLE COMPRISING A NON-METALLIC MAT OF HIGH TEMPERATURE RESISTANT MATERIAL ROLLED TO FORM A TUBE WITH THE MAT IMPREGNATED WITH A SULFATE BINDER SOLUTION AND COATED WITH A THERMO INSULATING MATERIAL. A SUPPORTING CONDUIT MAY BE USED TO PROVIDE PHYSICAL SUPPORT FOR THE ROLLED MAT.

Feb. 2 1971 N. H. STARK PROTECTIVE TUBE FOR ELECTRICAL ELEMENTS AND ITsMETHOD OF MANUFACTURE Filed Dec. 14, 1967 INVENTOR,

NORMAN H. STARK WHEELER; WHEELER,

HOUSE 8 CLEMENCY United States Patent 3,560,282 PROTECTIVE TUBE FORELECTRICAL ELEMENTS AND ITS METHOD OF MANUFACTURE Norman H. Stark, 8810Bonniwell Road, Mequon, Wis. 53092 Filed Dec. 14, 1967, Ser. No. 690,566Int. Cl. C04b 31/06 US. Cl. 15669 4 Claims ABSTRACT OF THE DISCLOSUREDisclosed herein is a protective tube for a thermocouple comprising anon-metallic mat of high temperature resistant material rolled to form atube with the mat impregnated with a sulfate binder solution and coatedwith a thermo insulating material. A supporting conduit may be used toprovide physical support for the rolled mat.

BACKGROUND OF THE INVENTION Thermocouples, which are used to indicatethe temperatures of molten metal, must be shielded in order to protectthe thermocouple elements while they are immersed in the molten metal.Protective tubes of high cost materials have been used; however, thesetubes are fragile and/or the abrasive wash action of aluminum and othernon-ferrous metals in melt operations tends to destroy the tube at arate that makes them uneconomical to use. Plain cast iron tubes havebeen resorted to due to the relatively fast destruction of the specialprotective tubes. These tubes are inexpensive but require frequentreplacement.

SUMMARY OF THE INVENTION This invention relates to an improved hightemperature protective tube for a thermocouple or other high temperatureoperating device. The tube includes a nonmetallic mat which has beenimpregnated with a sulfate solution and dried to stiffen the matso thatit can be rolled to a tubular form without unravelling. The mat iswrapped around the outer surface of a supporting structure which may bein the form of a steel conduit with a part of the mat extending beyondone end of the conduit. The mat is impregnated with a sulfate solutionwith an inorganic filler, such as kaolin, added to form a binder. Thesulfate-binder solution is dried and increases the temperatureresistance of the mat and when desirable bonds the mat to the supportingstructure. The sulfatebinder solution makes it possible to use an Egrade fiber glass rather than a special high temperature of S gradefiber glass.

The end of the rolled mat extending beyond the conduit has to be closedin order to protect any electrical elements in the inside of the tube.It has been found that where special plugs are used to close the end ofthe supporting conduit, they tend to leak due to the difference ofthermal coefficient of expansion. To prevent such leaks the materialused for the non-metallic mat is also used to make a plug to close oneend of the conduit and thereby prevent leaks which might result fromvariations in the thermal coefficient of expansion when using specialplugs for sealing the end of the tube.

Since the non-metallic mat and the metal supporting tube, when used,have different temperature characteristics, the cool down rate of thenon-metallic mat had to be reduced in order to prevent fracture due tothermoshock on withdrawal of the tube from the molten metal. A coatingof thermo insulating material is applied to the outer surface of thesulfate impregnated non-metallic mat to insulate the mat and retard cooldown. Since the sulfate has a tendency to precipitate the heatinsulating "ice materials, the surface of the sulfate impregnated mat isneutralized prior tocoating with the insulating material. This type ofprotective tube has been found to be of sufiicient strength to be usedfor high temperature use and to withstand accidental impact.

Other objects and advantages of the present invention will be apparentfrom the following detailed description when read in connection with theaccompanying drawing in which the single figure is a side view partly insection of the protective tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT Although the disclosure hereofis detailed and exact to enable those skilled in the art to practice theinvention, the physical embodiments herein disclosed merely exemplifythe invention which may be embodied in other specific structure. Thescope of the invention is defined in the claims appended hereto.

Referring to the figure in the drawing, the protective tube includes asupporting structure 10 such as a thin walled steel conduit ofsufficient length to enclose a pair of thermocouple wires or elements 8.The supporting structure or conduit may be of any length required for aparticular application and may have any outer configuration such assquare, triangular, etc. In the disclosed embodiment, a tubular conduithaving a wall thickness of approximately inch is shown. If thesupporting structure is to be removed from the-mat, it should be made ofa material that will not adhere to the mat during the steps required toform the tube. The conduit is covered with a non-metallic mat 16 offiber glass, crocidilite asbestos or a vitreous woven fiber such asRefrasil made by Hitco of California, which has been immersed in asulfate solution and dried with the end 18 of the mat extending beyondthe end of the conduit. The mat is then impregnated with asulfate-binder solution and allowed to dry until it is tacky.

A metal cap 12 is provided in the end of the conduit and a disc 15formed of the same non-metallic material that is used to cover theconduit is inserted in the extended end 18 of the fiber glass. The matis then folded over or crimped to the disc to close the end of the tube.Additional coatings of sulfate-binder solution may be added if thesupporting structure is to be removed.

A coating or layer 20 of a heat insulating solution such as a silicatecement is applied to the outer surface of the fiber glass to reduce therate of cool down of the fiber glass. This coating should beapproximately inch thick but can be varied as required. If the tube isto be used for extremely high temperature operations, the thickness ofthe heat insulating coating should be increased.

The woven mat may be a fiber glass material made from an E grade fiberglass which is used for electrical application. B grade being aclassification of fiber glass sold by the Owens-Corning Glass Company.This type of fiber glass is considered to be a low temperature materialand, therefore, not as expensive as a high temperature fiber glass suchas the 5 grade. A fiber glass mat of randomly arranged fiber glass couldalso be used. Other high temperature materials which could be used arecrocidilite asbestos, Refrasile and Fiberfrax made by the CarborundumCompany. All of these materials are capable of being impregnated with asulfate binder solution.

The process or method for manufacturing the protective tube shown in thefigure in the drawing includes the following steps:

(a) A strip of Woven fiber glass made from E grade fiber glass isinitially passed through a solution containing 5 to 50 percent aluminumsulfate (Al SO and dried.

3 The fiber glass is heated at a temperature sufiicient to dry thesolution. B staging of the sulfate solution provides sufiicient rigidityin the mat to prevent unravelling from occurring when the mat is cut.

(b) The dried mat is cut to the required dimensions so that it can beWrapped around the thin walled steel conduit with the end 18 of the matextending beyond the end 14 of the conduit. The disc is also cut fromthe strip of fiber glass at this time.

(c) The cut strip of dried fiber glass is impregnated with asulfate-binder solution consisting of aluminum sulfate (Al SO kaolin andwater. The range of the materials used for this impregnant is believedto be critical to the success of the protective tube. Optimum resultswere obtained with a solution (by weight) containing 62 /2 percent Al SOand 37 /2 percent kaolin diluted to 200 percent with water. The amountof water can be varied to effect a viscosity suflicient to allow forcomplete impregnation of the woven fiber glass. The impregnated fiberglass material used to form the disc 15 was also impregnated with thesame sulfate solution and dried prior to insertion in the end 18 of themat. The ratio of Al SO to kaolin may be varied to 60 to 65 percent A1SO to to 35 percent kaolin. Tests have indicated that if the aluminumsulfate is increased to 70 percent or more or is reduced below 58percent the heat resistance of the fiber glass drops off considerably.Other inorganic fillers such as talc may be used in place of the kaolin.

(d) The impregnated fiber glass, While still Wet, is Wrapped around theconduit with the end 18 of the fiber glass extending beyond the end 140f the conduit. The sulfate impregnated fiber glass disc 15 is insertedinto the extended end of the rolled mat in the end 14 of the conduit inabutting engagement with the metal cap 12. The fiber glass, after beingwrapped on the conduit, is air dried for approximately four hours toallow the sulfatebinder solution to dry to a tacky condition. The end 18of the fiber glass is then crimped over the disc 15 and will adhere tothe disc due to the tacky condition of the fiber glass.

(f) The rolled tube is placed in an oven at a temperature of 200 F. andis left in the oven for approximately one hour. The temperature is thenraised to 275 F. and drying continued for about two hours.

(g) The tube is removed from the oven and is washed with warm Water toremove excess binder from the surface of the fiber glass.

(h) Since the sulfate binder is an acid material which precipitatessilicates, the surface of the tube must be neutralized prior to coatingthe outside of the tube with the thermo insulating material. This isdone by wiping or dipping the tube into a solution of 20 percent causticsoda to neutralize the sulfate binder on the outside surface of theWoven fiber glass. Soda, ash or any other alkalis can be used toneutralize the mat.

(i) The thermo insulating material such as silicate cement 20 is thenapplied to the outer coating of the tube and dried. While varioussilicates may be used, in the embodiment described, a silicate cementcalled Atmoset formulated by the A & P Green Company was used.Additional coatings of the silicate cement can then be added by merelybrushing them onto the surface of the tube until the desired thicknessis obtained. A thickness of approximately ,4 inch is consideredsufficient for this tube.

A high temperature insulating coating material such as a Cera-Kote madeby Johns-Manville could also be used.

What is claimed is: 1. A protective tube for a thermocouple comprising ahollow tubular mat of high temperature resistant fiber glass materialclosed at one end,

said material being impregnated with an aluminum sulfate containingbinder, the outer surface of said mat beiing neutralized with a solutioncontaining caustic so a,

and a layer of thermo insulating silicate material coated on the outsidesurface and the closed end of said mat.

2. A protective tube according to claim 1 including a thin walled steelsupporting conduit within said hollow tubular mat.

3. A protective tube according to claim 1 wherein said sulfate binderincludes, by weight, to percent aluminum sulfate and 35 to 40 percentinorganic filler.

4. A protective tube according to claim 2 including a disc of hightemperature material formed from the same material as the tubular matclosing the said one end of the conduit.

References Cited UNITED STATES PATENTS 2,3 84,024 9/1945 Goller 136234X2,405,075 7/1946 Vollrath 1136-234X 2,725,226 11/1955 Shea 106-70XBENJAMIN R. PADGETT, Primary Examiner S. HELLMAN, Assistant ExaminerU.S. Cl. X.R.

